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Subject: Re: [PATCH v2] tools/memory-model: Clarify LKMM's limitations in
 litmus-tests.txt
From:   =?utf-8?Q?Paul_Heidekr=C3=BCger?= <Paul.Heidekrueger@in.tum.de>
In-Reply-To: <20220711163011.GN1790663@paulmck-ThinkPad-P17-Gen-1>
Date:   Tue, 12 Jul 2022 08:45:51 +0200
Cc:     Alan Stern <stern@rowland.harvard.edu>,
        Marco Elver <elver@google.com>,
        Andrea Parri <parri.andrea@gmail.com>,
        Will Deacon <will@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Boqun Feng <boqun.feng@gmail.com>,
        Nicholas Piggin <npiggin@gmail.com>,
        David Howells <dhowells@redhat.com>,
        Jade Alglave <j.alglave@ucl.ac.uk>,
        Luc Maranget <luc.maranget@inria.fr>,
        Akira Yokosawa <akiyks@gmail.com>,
        Daniel Lustig <dlustig@nvidia.com>,
        Joel Fernandes <joel@joelfernandes.org>,
        LKML <linux-kernel@vger.kernel.org>,
        linux-arch <linux-arch@vger.kernel.org>,
        Charalampos Mainas <charalampos.mainas@gmail.com>,
        Pramod Bhatotia <pramod.bhatotia@in.tum.de>,
        Soham Chakraborty <s.s.chakraborty@tudelft.nl>,
        Martin Fink <martin.fink@in.tum.de>
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To:     "Paul E. McKenney" <paulmck@kernel.org>
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Paul E. McKenney <paulmck@kernel.org> wrote:

> On Mon, Jul 11, 2022 at 05:14:55PM +0200, Paul Heidekr=C3=BCger wrote:
>>> On 8. Jul 2022, at 20:47, Paul E. McKenney <paulmck@kernel.org> =
wrote:
>>>=20
>>> On Fri, Jul 08, 2022 at 10:45:06AM -0400, Alan Stern wrote:
>>>> On Fri, Jul 08, 2022 at 01:44:06PM +0200, Marco Elver wrote:
>>>>> On Tue, 14 Jun 2022 at 17:49, Paul Heidekr=C3=BCger
>>>>> <paul.heidekrueger@in.tum.de> wrote:
>>>>>> As discussed, clarify LKMM not recognizing certain kinds of =
orderings.
>>>>>> In particular, highlight the fact that LKMM might deliberately =
make
>>>>>> weaker guarantees than compilers and architectures.
>>>>>>=20
>>>>>> Link: =
https://lore.kernel.org/all/YpoW1deb%2FQeeszO1@ethstick13.dse.in.tum.de/T/=
#u
>>>>>> Signed-off-by: Paul Heidekr=C3=BCger =
<paul.heidekrueger@in.tum.de>
>>>>>> Co-developed-by: Alan Stern <stern@rowland.harvard.edu>
>>>>>=20
>>>>> Reviewed-by: Marco Elver <elver@google.com>
>>>>>=20
>>>>> However with the Co-developed-by, this is missing Alan's SOB.
>>>>=20
>>>> For the record:
>>>>=20
>>>> Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
>>>>=20
>>>> (Note that according to =
Documentation/process/submitting-patches.rst,=20
>>>> the submitting author's SOB is supposed to come last.)
>>>=20
>>> And this is what I ended up with. Please provide additional feedback
>>> as needed, and in the meantime, thank you all!
>>>=20
>>> 							Thanx, Paul
>>=20
>> Looks great - my first commit in the Linux kernel!
>=20
> Congratulations!!! ;-)

Thanks! Hopefully many more to come :-)

> My commits for the upcoming merge window, which is probably 2-3 weeks
> from now, are already set. So this is targeted at the merge window
> after that, which is likely to be in late September or early October.
>=20
> So it is well on its way!

Awesome!

Many thanks,
Paul

> 							Thanx, Paul
>=20
>> Thanks everyone!
>>=20
>> Paul
>>=20
>>> =
------------------------------------------------------------------------
>>>=20
>>> commit 3c7753e959706f39e1ee183ef8dcde3b4cfbb4c7
>>> Author: Paul Heidekr=C3=BCger <paul.heidekrueger@in.tum.de>
>>> Date: Tue Jun 14 15:48:11 2022 +0000
>>>=20
>>> tools/memory-model: Clarify LKMM's limitations in litmus-tests.txt
>>>=20
>>> As discussed, clarify LKMM not recognizing certain kinds of =
orderings.
>>> In particular, highlight the fact that LKMM might deliberately make
>>> weaker guarantees than compilers and architectures.
>>>=20
>>> Link: =
https://lore.kernel.org/all/YpoW1deb%2FQeeszO1@ethstick13.dse.in.tum.de/T/=
#u
>>> Co-developed-by: Alan Stern <stern@rowland.harvard.edu>
>>> Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
>>> Signed-off-by: Paul Heidekr=C3=BCger <paul.heidekrueger@in.tum.de>
>>> Reviewed-by: Marco Elver <elver@google.com>
>>> Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>
>>> Cc: Charalampos Mainas <charalampos.mainas@gmail.com>
>>> Cc: Pramod Bhatotia <pramod.bhatotia@in.tum.de>
>>> Cc: Soham Chakraborty <s.s.chakraborty@tudelft.nl>
>>> Cc: Martin Fink <martin.fink@in.tum.de>
>>> Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
>>>=20
>>> diff --git a/tools/memory-model/Documentation/litmus-tests.txt =
b/tools/memory-model/Documentation/litmus-tests.txt
>>> index 8a9d5d2787f9e..cc355999815cb 100644
>>> --- a/tools/memory-model/Documentation/litmus-tests.txt
>>> +++ b/tools/memory-model/Documentation/litmus-tests.txt
>>> @@ -946,22 +946,39 @@ Limitations of the Linux-kernel memory model =
(LKMM) include:
>>> 	carrying a dependency, then the compiler can break that =
dependency
>>> 	by substituting a constant of that value.
>>>=20
>>> -	Conversely, LKMM sometimes doesn't recognize that a particular
>>> -	optimization is not allowed, and as a result, thinks that a
>>> -	dependency is not present (because the optimization would break =
it).
>>> -	The memory model misses some pretty obvious control dependencies
>>> -	because of this limitation. A simple example is:
>>> +	Conversely, LKMM will sometimes overestimate the amount of
>>> +	reordering compilers and CPUs can carry out, leading it to miss
>>> +	some pretty obvious cases of ordering. A simple example is:
>>>=20
>>> 		r1 =3D READ_ONCE(x);
>>> 		if (r1 =3D=3D 0)
>>> 			smp_mb();
>>> 		WRITE_ONCE(y, 1);
>>>=20
>>> -	There is a control dependency from the READ_ONCE to the =
WRITE_ONCE,
>>> -	even when r1 is nonzero, but LKMM doesn't realize this and =
thinks
>>> -	that the write may execute before the read if r1 !=3D 0. (Yes, =
that
>>> -	doesn't make sense if you think about it, but the memory model's
>>> -	intelligence is limited.)
>>> +	The WRITE_ONCE() does not depend on the READ_ONCE(), and as a
>>> +	result, LKMM does not claim ordering. However, even though no
>>> +	dependency is present, the WRITE_ONCE() will not be executed =
before
>>> +	the READ_ONCE(). There are two reasons for this:
>>> +
>>> + The presence of the smp_mb() in one of the branches
>>> + prevents the compiler from moving the WRITE_ONCE()
>>> + up before the "if" statement, since the compiler has
>>> + to assume that r1 will sometimes be 0 (but see the
>>> + comment below);
>>> +
>>> + CPUs do not execute stores before po-earlier conditional
>>> + branches, even in cases where the store occurs after the
>>> + two arms of the branch have recombined.
>>> +
>>> +	It is clear that it is not dangerous in the slightest for LKMM =
to
>>> +	make weaker guarantees than architectures. In fact, it is
>>> +	desirable, as it gives compilers room for making optimizations.=20=

>>> +	For instance, suppose that a 0 value in r1 would trigger =
undefined
>>> +	behavior elsewhere. Then a clever compiler might deduce that r1
>>> +	can never be 0 in the if condition. As a result, said clever
>>> +	compiler might deem it safe to optimize away the smp_mb(),
>>> +	eliminating the branch and any ordering an architecture would
>>> +	guarantee otherwise.
>>>=20
>>> 2.	Multiple access sizes for a single variable are not supported,
>>> 	and neither are misaligned or partially overlapping accesses.